Hi Sudee
Just a thought - have you looked at Warwick Bridge Roman Catholic register which start well before this date, and also the Quakers. I think most of the other non-conformist registers are later.
Also, have you looked for baptisms for the osiblings in case just an odd one has been missed. Is that wherre you got the other children from? If there's only John missing, and p[arents are from Northumberlasnd, could they havr gone back to visit, or nipped over the border into Annandale?
I don't know how wide you've spread your search either, but with Cumbria being so rural, most people were not living next to the parish church for the parish named in the census and may well have been baptised quite a long way away, especially if the father was a farm worker and moved between censuses.
By the way, was the 1811 on another child's baptism?
Just a few thoughts.

Best wishes
Emms